In wireless communications systems of the art, a variety of transmission protocols have been developed for providing airlink service to users. Exemplary network services based on such transmission protocols include High Rate Packet Data (HRPD), Universal Mobile Telecommunications System (UMTS) and Long Term Evolution (LTE)—sometimes referred to in terms of its air interface Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). Each of these network services is defined in terms of a particular Radio Access Technology (RAT), and, in general, the RAT defining each different transmission protocol requires a unique RF configuration for transmission and reception of communications based on a given RAT. A multi-mode user terminal having RF capabilities compatible with two or more RATs can switch to an available RAT that is most suitable at the relevant time. Given availability of such multi-mode terminals, an operator can also do load-sharing between different RAT carriers provided in its network, so as to improve network usage and performance.
As implied by the prior paragraph, multi-mode terminals are mobile terminals that have the capability to access two or more RAT networks. In the circumstance of such a multi-mode terminal having simultaneous available access to two different RAT networks, or the mobile terminal roaming from a carrier of one RAT type toward another carrier of another RAT type—e.g., from an LTE network to an HRPD network, system algorithms, generally called Multi-Mode System Selection (MMSS), provide the mobile terminal with a method for finding a carrier suited to the services subscribed to by the mobile terminal. Specifically, MMSS enables a mobile terminal to select an authorized or preferred system (and/or a carrier in that system) when the mobile powers up or moves out of the current serving system coverage, such as roaming to another system of a different operator.
With MMSS, a system operator will provision in the mobile a list of potentially available carriers with associated access priorities (e.g., a Preferred Roaming List (PRL) in CDMA/HRPD systems) for the mobile terminals which are subscribed to the operator. At the same time, however, mobiles engaged in idle reselection (or idle handoff (HO)) among the carriers of different RATs may experience different carrier selection criteria, such as prioritization among neighbor-list carriers broadcast by an access network (AN). Note that the neighboring carriers may be from different operators with different Public Land Mobile Network IDs (PLMN_IDs).
When these two mechanisms (MMSS and idle reselection) are both implicated for a given mobile carrier-selection event, the priorities specified in each mechanism may conflict with one another. This is likely to cause the affected mobile terminal to experience ping-ponging between the carriers of different RATs and different operators.
In addition, when cross operator roaming is triggered, a mobile terminal will often expend a large amount of power in search of a roaming handoff, and a large delay will be experienced by a mobile terminal to complete the handoff.